Parkinson's disease (PD) is an age-related neurodegenerative disorder characterized by resting tremor, rigidity and bradykinesia. These clinical features are thought to arise from reduced dopaminergic input to the striatum, which is caused by the degeneration of dopaminergic neurons in the substantia nigra. The occurrence of PD is largely sporadic, but clinical syndromes resembling sporadic PD have been linked to mutations in at least 5 distinct genes (1-synuclein, parkin, DJ-1, PINK1 and LRRK2). Elucidation of the pathogenic mechanisms underlying the selective dopaminergic degeneration in familial parkinsonism will likely provide important clues to the pathogenic mechanisms responsible for idiopathic PD. The recessive inheritance mode of the mutations and the existence of large deletions in the parkin, DJ-1 and PINK1 genes indicate a loss-of-function pathogenic mechanism. Our previous generation and multidisciplinary analysis of parkin-/-, DJ-1-/- and PINK1-/- mice have demonstrated that each of these gene products is required for normal dopaminergic function and evoked dopamine (DA) release in nigrostriatal terminals. Our recent work showed that LRRK2 R1441C knockin (KI) mice exhibit similar defects in dopaminergic neurotransmission and DA release. Thus, presynaptic dopaminergic dysfunction may be a central converging pathogenic mechanism for PD. Our studies also showed that loss of PINK1 or Parkin impairs mitochondrial function, suggesting that impairment of mitochondrial respiration in the absence of PINK1 or Parkin may underlie the DA release defect. In this competing renewal application, we propose two Specific Aims to continue our investigation of PD pathogenic mechanisms. First, we will elucidate the mechanism underlying mitochondrial respiration defects caused by loss of PINK1 or Parkin, and determine whether mitochondrial calcium homeostasis and cell death susceptibility are affected. Second, we will further explore dopamine neurotransmission in vivo and determine whether the defect in evoked DA release is caused by disrupted calcium homeostasis and whether loss of PINK1 or Parkin renders cells more sensitive to mitochondrial inhibition. The completion of the proposed study will provide significant insight into the biology of PINK1 and Parkin as well as their regulation of DA release, and will determine whether disruption of calcium homeostasis is a key unifying event in DA neurotransmission, mitochondrial function and cell death susceptibility. Our long-term goal is to understand the pathogenic mechanism underlying dopaminergic dysfunction and degeneration, and to characterize the cellular and molecular pathways responsible for PD pathogenesis.